Coking is one of the older refining processes. The purpose of a delayed coking plant is to convert heavy residual oils (e.g. tar, asphalt, etc.) into lighter, more valuable motor fuel blending stocks. Refinery coking is controlled, severe, thermal cracking. It is a process in which the high molecular weight hydrocarbon residue (normally from the bottoms of the vacuum flasher in a refinery crude unit) are cracked or broken up into smaller and more valuable hydrocarbons.
Coking is accomplished by subjecting the feed charge to an extreme temperature of approximately 930° F. that initiates the cracking process. The light hydrocarbons formed as a result of the cracking process flash off and are separated in conventional fractionating equipment. The material that is left behind after cracking is coke, which is mostly carbon. In addition to coke, which is of value in the metal industry in the manufacture of electrodes, fuel coke, titanium dioxide, etc., the products of a delayed coking plant include gas (refinery fuel gas), liquefied petroleum gas, naphtha, light gas oil, and heavy gas oil.
Most of the world's coking capacity is generated by delayed coking processes. Delayed coking can be thought of as a continuous batch reaction. The process makes use of paired coke drums. One drum (the active drum) is used as a reaction vessel for the thermal cracking of residual oils. This active drum slowly fills with coke as the cracking process proceeds. While the active drum is being filled with coke, a second drum (the inactive drum) is in the process of having coke removed from it. The coke drums are sized so that by the time the active drum is filled with coke, the inactive drum is empty. The process flow is then switched to the empty drum, which becomes the active drum. The full drum becomes the inactive drum and is emptied or decoked. By switching the process flow back and forth between the two drums in this way, the coking operation can continue uninterrupted.
In operation, after being heated in a direct-fired furnace, the oil is charged to the bottom of the active coke drum. The cracked light hydrocarbons rise to the top of the drum where they are removed and charged to a fractionator for separation. The heavier hydrocarbons are left behind, and the retained heat causes them to crack to coke.
In FIG. 1, a schematic diagram illustrates one example of a delayed coking closed blowdown system (hereinafter “delayed coking quench system”), where the effluent from the inactive drum is processed. The quenching of the inactive coke drum produces large quantities of steam with some hydrocarbons which are processed in this system.
A quench tower 106, a blowdown condenser 122 and a settling drum 124 form a closed blowdown system, which is used to recover effluent from the coke drum steaming, quenching and warming operations.
In conventional systems, a blowdown header line 104 communicates the hot vapor from a coke drum overhead line 101 to a quench tower 106 during the steaming and water quenching operation.
Just upstream of the quench tower 106, the hot vapor is quenched by a controlled injection of water from the process. During the water quenching operation, the overhead stream from the quench tower 106, is substantially steam with small amounts of hydrocarbons, and is sent in an overhead line 120 to the blowdown condenser 122.
The blowdown condenser 122 condenses the bulk of the overhead stream to form a blowdown condenser outlet stream which is communicated in the blowdown condenser outlet stream line 123 to a blowdown settling drum 124.
In the settling drum 124, the blowdown condenser outlet stream is separated into a sour water stream 126, a light slop oil stream 132 and a hydrocarbon vapor stream 127. The hydrocarbon vapor stream 127 is sent to the blowdown ejector 158 and then to the fractionator overhead system 160. The light slop oil stream 132 is returned to the quench tower 106. The blowdown ejector 158 is used to reduce the pressure in the closed blowdown system and coke drum at the end of the water quench prior to isolating a coke drum and venting the coke drum to atmosphere. Alternatively, a compressor may be used in place of a blowdown ejector 158. The blowdown ejector, which may be steam-driven, is used to target 2 psig before venting the drum to atmosphere. Effluent from blowdown ejector 158 is sent to the fractionator overhead system 160, and recovered to the main process.
A quench water tank 140 is used to provide water to quench water line 148 and to the coke cutting line 142.
During the quench operation the inactive coke drum is connected to the closed blowdown system and the pressure in the inactive coke drum is essentially the same as the pressure in the closed blowdown system. At the end of the quench operation, the inactive coke drum is isolated from the closed blowdown system and is vented to the atmosphere. An ejector or small compressor may be used in a line containing the hydrocarbon vapor stream 127 to reduce the pressure in the closed blowdown system and inactive coke drum to about 2 psig or less prior to isolating and venting the inactive coke drum as required by current environmental regulation guidelines. Despite venting the inactive coke drum to the atmosphere at 2 psig, a plume of steam is produced that may contain hydrocarbon vapors (e.g. methane, ethane, hydrogen sulfide) and coke fines (hereinafter collectively “atmospheric emissions”). Maintaining a pressure of 2 psig in the inactive coke drum prior to venting to the atmosphere is also an issue because the coke drum pressure can spike due to continuing heat evolution from the coke bed after isolation from the closed blowdown system. On some older units, which start to vent at around 15 psig, noise is also a significant issue.
It is known that a delayed coking quench system may be modified to include a coke drum quench overflow system to provide the benefit of overflowing a coke drum at the end of the quench operation. Existing overflow systems are varied and some have been known to generate undesirable odors, and gas releases or fires, plugging exchangers and residual coke fines in lines that are flushed into other equipment when the coke drums are returned to the fill cycle because the overflow stream can contain significant atmospheric emissions. In addition, many existing overflow systems do not minimize atmospheric emissions, and merely relocate the source of the atmospheric emissions.
Because some existing overflow systems have American Petroleum Institute (“API”) separators or other equipment open to the atmosphere, there can be atmospheric emissions, which is a serious problem. When the overflow stream is sent through an air cooler without being properly filtered, the air cooler can plug, which is also a problem in some existing overflow systems. In parts of the piping system used by existing overflow systems, coke fines are often left after the overflow operation, which are then flushed into the quench tower or fractionator when returning to the normal valving arrangement. A delayed coking unit that produces shot coke can result in larger amounts of oil and coke fines in the quench overflow stream, which is more problematic to handle.